Moreover, a low-height flight trajectory such as this is made safe with respect to the terrain, that is to say it is formed in such a way that an aircraft which follows this trajectory presents a risk of collision with the terrain overflown (that is to say directly with the ground, or else with a work or an element situated on the ground) which is almost zero, and in any event presents a probability of less than a very low predetermined value, for example 10−9. Consequently, if the aircraft is guided along a low-height flight trajectory such as this, any collision with the terrain overflown is highly improbable.
It is known that during a low-height flight in reduced visibility, cases of emergency (fault or operational requirement for example) may arise for which it is necessary to interrupt the conduct of the low-height flight and to bring the aircraft, definitively or temporarily, to a high altitude where the aircraft is safe, sheltered from a collision with the terrain (ground, work situated on the ground). This altitude is called the safety altitude. A method for determining such a safety altitude is known from document FR-04 05385.
A climb phase intended to bring the aircraft from the low-height flight trajectory to the safety altitude is not without risk. Specifically, for example climbing in a straight line, in conditions of reduced visibility, is not a satisfactory procedure, since the low-height flight is optimized to pass through the bottoms of valleys, as close as possible to the terrain overflown, and so climbing blindly in a straight line is very risky.
Moreover, the safety altitude is generally dependent on a certain geographical zone and therefore has local and limited validity. This is all the truer if this zone is small and dependent on the low-height flight trajectory, such as for example for the safety altitude determined by the method described in the aforesaid patent application FR-04 05385. In the course of the flight, the safety altitude therefore frequently varies.
Additionally, in particular in the military field, the interruption of a low-height flight is sometimes carried out in a great emergency so that any additional task requested of the crew of the aircraft, such as manually selecting a safety altitude value for example, can constitute an excessive workload in an emergency situation such as this.